Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices. CFMs include implantable pacemakers, implantable cardioverter defibrillators (ICDs), and devices that include a combination of pacing and defibrillation including cardiac resynchronization therapy. The devices are typically used to treat patients using electrical therapy and to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include electrical leads in communication with sense amplifiers to monitor electrical heart activity within a patient, and often include sensors to monitor other internal patient parameters. Other examples of implantable medical devices include implantable insulin pumps or devices implanted to administer drugs to a patient.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs are able to detect abnormally slow heart rate, or bradycardia. Additionally, the IMDs are able to detect abnormally rapid heart rate, or tachyarrhythmia. Although detecting an occurrence of tachyarrhythmia is important, it is more helpful if additional physiologic information is known about the arrhythmia, such as if the arrhythmia is hemodynamically stable or unstable. The IMDs are further able to detect differences in time between contractions of the left and right ventricles and to provide pacing therapy to resynchronize contractions of the left and right ventricles. However, proper resynchronization does not necessarily involve merely pacing the left and right ventricles simultaneously. Instead, there is a need to pace in a manner that improves the efficacy of the contractions. The present inventors have recognized a need for improved sensing of events related to cardiac activity.